Can you pin a king with 2 knights and a king ?

if you mean forced mate the king with two knights? No, you can't. It is called the great injustice of chess. :) having 6 more points of material against your opponent and not being able to mate is very sad. :)

Knights can't "pin" anything, and while pieces can be "pinned to" the King, the King cannot be pinned at all, of course..

Two Knights alone cannot force mate against a lone King, although the King can blunder into being mated. The forcing sequence requires stalemating the King for one move, which is a draw. So the only way the two Knights can force mate is if the King has a single pawn to move when he cannot.

The great endgame composer A.A. Troitzky did an entire treatise on the subject, exploring which pawns would work and how far advanced they could be and still allow the Knights time to win before they promote and start checking the other King. As it turned out, his work was so accurate there were no corrections needed when computer "tablebases" figured out the possibilities.

The treatise was published as an appendix to some editions of his 360 Brilliant and Instructive Endings, which is highly recommended with or without the appendix.